The present invention relates to a method and system for scrambling transmitted information and, more particularly, to a method and system for scrambling and unscrambling the transmissions of information services such as subscription television to prevent the unauthorized use of such services.
The secure transmission of information has become extremely important, particularly in the field of television transmission via cable, satellite and other media. Numerous encoding or scrambling techniques have been developed and some are now in use, particularly in broadcast systems where there is no control over who receives the signals and the signals must be encoded to prevent unauthorized use of the received signals.
One known broadcast subscription television system transmits a video signal that has ben amplitude modulated by a sine wave signal in such a way that the blanking and synchronizing levels cannot be recognized by a normal television receiver. Specifically, the video signal is modulated in amplitude by a sine wave synchronized with the horizontal line rate of the video. The sine wave amplitude modulates the synchronizing signals and the video signal so that they are at the same level other than that which the television receiver expects. Thus, the television receiver synchronizes on something other than the normal synchronizing signals. The resulting video display on the normal television receiver is unintelligible or at least very annoying to a viewer.
While this method is relatively inexpensive and effective, it can be easily defeated and cannot be made secure. The sine wave is at a fixed rate and its amplitude modulation effects can be relatively easily eliminated at the receiver by unauthorized individuals. Since there is no way to vary the scrambling, a fixed circuit arrangement which, in effect, reverses the amplitude modulation, defeats the security of the system.
Other approaches to scrambling have proven more secure but are typically more expensive due to their complexity. In addition, certain other scrambling methods have affected picture quality in an unacceptable manner, and attempts to improve picture quality have resulted in the lowering of the level of security.